Transmission schemes such as Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) schemes (herein after referred to as OFDM) have come to attention in these years.
FIG. 1 is a block diagram illustrating part of processing performed in a transmitter using a transmission scheme such as OFDM.
In FIG. 1, an OFDM symbol generator 1 includes an inverse fast Fourier transform (IFFT) section 11 and a cyclic prefix (CP) adding section 12. The IFFT section 11 performs inverse fast Fourier transform on multiple transmitter signals Sf at a time to generate an effective symbol Xt which is a time-domain signal including multiple sub-carriers that are orthogonal to each other in frequency.
FIGS. 2A, 2B and 2C illustrate a signal generated by the processing illustrated in FIG. 1 and its data. FIG. 2A is a schematic diagram of a time-domain signal and FIG. 2B illustrates a specific signal waveform corresponding to FIG. 2A. As illustrated in FIG. 2B, the signal is represented by discrete data strings allocated at even intervals in the time domain. Since an effective symbol Xt is a superposition of harmonic signals that have a period that is an integral submultiple of an effective symbol length due to the nature of inverse Fourier transform, signal data De1 and De2 at both ends of the effective symbol Xt have continuous phases.
As illustrated in FIG. 2A, the CP adding section 12 copies the tail end section of an effective symbol Xt and adds the tail section to the head end section of the effective symbol Xt as a cyclic prefix Xc to generate an OFDM symbol which is a unit of transmission in the OFDM scheme. In this way, the OFDM symbol generator 1 sequentially generates OFDM symbols X1, X2, . . . in which cyclic prefixes Xc1, Xc2, . . . are added to effective symbols Xt1, Xt2, . . . as illustrated in FIG. 2C for multiple frequency-domain signals Sf.
A cyclic prefix CP, also known as guard interval, is a redundant signal added in order to reduce interference with a delay wave due to multipath propagation on a transmission line and is removed at a receiving end.
FIGS. 3A to 3D illustrate the relationship between signal processing in an upsampling processor 2 and digital-to-analog (D-A) conversion. The upsampling processor 2 performs interpolation on time-domain discrete data included in the OFDM symbol X illustrated in FIG. 3A to increase the number of pieces of signal data (the number of samples), thereby increasing the sampling frequency of OFDM symbol X.
FIG. 3B illustrates an analog signal waveform resulting from D-A conversion of the OFDM symbol X depicted in FIG. 3A by a D-A converter 3. If the sampling frequency of the OFDM symbol X is low, a hold period Th1 will be long, which generates distortions and high-frequency components in the frequency spectrum of the OFDM symbol X. In order to reduce such distortions and components, the upsampling processor 2 needs to perform processing for increasing the sampling frequency of the OFDM symbol X.
The upsampling processor 2 includes an upsampling section 21 and a filter circuit 22.
Specifically, the upsampling section 21 inserts 0 points, p1, p2, . . . as additional sampling points between sampling points of the OFDM symbol X as illustrated in FIG. 3C. For example, inserting three 0 points between sampling points will increase the sampling frequency by a factor of 4. The OFDM symbol X including the 0 points inserted as illustrated in FIG. 3C is subjected to filtering by the low-pass filter circuit 22 to remove high-frequency components from the OFDM symbol X. As a result, the OFDM symbol X with appropriately interpolated points as illustrated in FIG. 3D is generated. One approach to implementing the filtering is a Finite Impulse Response (FIR) filter that performs convolution operations on a sampled signal in continuous periods and a filtering coefficient. A filter that performs such convolution operations will be hereinafter referred to as a convolution filter.
Since the time period Th2 between sampling points of the OFDM symbol X is reduced, the hold period Th2 is short even after the D-A conversion. Accordingly, distortions and high-frequency components in the frequency spectrum of the D-A converted OFDM symbol X are suppressed. Then, the OFDM symbol X is converted by a high-frequency circuit RF to an OFDM transmission signal and is transmitted through an antenna 4.
On the other hand, the number of inverse Fourier transform points may be increased to increase the bandwidth for the transform to increase the sampling frequency of the transformed effective symbol Xt. However, this increases the circuit size of the IFFT section 11. Therefore, in order to keep the circuit size small, upsampling needs to be performed in the transmitter.
Japanese Laid-Open Patent Publication No. 2003-134083, for example, describes the circumstances described above.
However, when symbols X1, X2, . . . are convolved using the upsampling in the filter circuit 22, data strings at the boundary between symbols to be convoluted include a data string of an adjacent symbol and therefore data interference occurs in the convolution operation, which may result in an error such as a bit error at a receiver.